1. Field of the Invention
The present invention relates in general to a cushioning device for a press, and more particularly to a method and an apparatus for establishing an optimum pneumatic pressure in a cushioning pneumatic cylinder adapted to bias a cushion platen in an upward direction.
2. Discussion of the Related Art
A press adapted to effect a drawing operation on a blank is generally equipped with a cushioning device which includes (a) a cushion platen supporting a pressure member and (b) a cushioning pneumatic cylinder for biasing the cushion platen in an upward direction. The cushioning device is constructed to apply a blank-holding force to the pressure member to hold the blank placed on the pressure member, based on a pneumatic pressure of the cushioning pneumatic cylinder. An example of a press equipped with such a cushioning device is disclosed in laid-open Publication No. 62-20711 (published in 1987) of unexamined Japanese Utility Model Application. Usually, the pneumatic pressure of the cushioning pneumatic cylinder is adjusted so as to obtain an intended product from the blank, in a trial-and-error procedure by performing test pressing operations each time a die set used on the press is changed.
However, the adjustment of the pneumatic pressure in the trial-and-error procedure as described above is not only cumbersome and time-consuming, but also requires a relatively high level of skill of the operator and suffers from difficulty in producing pressed articles with consistently high quality. In view of this drawback, there has been proposed a method of automatically adjusting the pneumatic pressure of the cushioning pneumatic cylinder to an optimum value suitable for the intended product, on the basis of die-set information indicative of specifications of a die set used, and machine information indicative of the specifications of the press in question, as disclosed in co-pending application No. 08/043,822. Described more specifically by reference to FIG. 1, an optimum value Pax of a pneumatic pressure Pa of a cushioning pneumatic cylinder 32 used in a press shown in the figure is calculated according to the following equation (1), and the pneumatic pressure Pa is adjusted to the calculated optimum value Pax by suitably controlling a pneumatic pressure control circuit 38 and a shut-off valve 37 while the actual pneumatic pressure Pa is detected by a pneumatic pressure sensor 39: EQU Pax=(Fso+Wa+Wr+n.multidot.Wp)/A (1)
where, Fso: optimum blank-holding force for an intended drawing operation, PA1 Wr: weight of a pressure ring 28 (pressure member), PA1 n: number of cushion pins 22, PA1 Wa: weight of cushion platen 26, PA1 Wp: weight of each cushion pin 22, PA1 A: pressure-receiving area of the cylinder 32.
The optimum blank-holding force Fso, weight Wr of the pressure ring 28 and number n of the cushion pins 22 used in the above equation (1) are determined for each die set used, by test pressing operations on a try press or test press (namely, a press on which test pressing operations are performed using a new die set, to find out optimum pressing conditions for obtaining an intended product).
When the die set is changed from one to another or when the new die set is installed on the press, the supply of compressed air to the cushioning pneumatic cylinder 32 and the adjustment of the pneumatic pressure of the cylinder 32 are conducted while the cushion platen 26 is placed at its lowermost position at which the upper die 18 is attached to a press slide 20. In other words, the pneumatic pressure adjustment is effected while the upper die 18 is attached to the press slide 20. The pneumatic pressure adjustment concurrently with the attachment of the upper die 18 to the press slide 20 advantageously shortens the overall time required for the changeover involving the attachment of the die set and the pneumatic pressure adjustment. On the other hand, however, a subsequent upward movement of the cushion platen 26 from its lowermost position to an operating position for a pressing operation will cause an increase in the volume of the pneumatic cylinder 32 and a resulting decrease in the pneumatic pressure. Consequently, the pneumatic pressure of the pneumatic cylinder should be raised by supplying an additional amount of compressed air to the pneumatic cylinder, prior to a pressing operation in which the lower stroke end of the cushion platen 26 is located above its lowermost position. This re-adjustment of the pneumatic pressure requires an additional time before starting the pressing operation. In other words, the commencement of the pressing operation is accordingly delayed due to the requirement for the re-adjustment of the pneumatic pressure.